1. Field of the Invention
The present invention pertains generally to the field of environmental sampling and, more particularly, to collection of core from a formation.
2. Description of Related Art
Sampling tubes are driven through soils and sediments that compose a formation to collect a core for further study and analysis. As the sampling tube is driven into the formation, a sampled core from the material of the formation collects inside the sampling tube. After the sampling tube has passed through and collected core from a desired profile of the formation and has reached a desired depth of sampling, the sampling tube is extracted to retrieve the collected sample. As the sampling tube is extracted from the formation, a borehole is created in the place of the collected core.
During the sampling of soils and sediments using a sampling tube, the vacuum created due to the extraction of the sampling tube may cause sample loss from the tube. After the conclusion of sampling, the borehole that remains open creates a path for potential travel of contamination across the formation. Thus, the creation of the borehole impacts both the collected core and the formation that is left behind.
While the sampling tube is being extracted, a partial vacuum is created in the open space below the tip of the sampling tube. The partial vacuum places suction on the tube and the collected core inside the tube. Depending on the material of the formation that is collected near the tip of the sampling tube, and depending on how tightly this material is held to the tube, the suction may suck out the sampled core and result in loss of sampled material. A compact core is unlikely to be sucked out but a sample collected from fine and lose sandy material is probably lost to the suction of the vacuum.
Loss of sample due to the vacuum that is generated below a retrieving sampling apparatus is accepted in the field of sampling. The sampling tubes are driven below a desired depth to compensate for the uncontrollable and, at times, inevitable loss of sample.
After the conclusion of sampling, the open borehole presents a cross-contamination pathway. Once the sampling tube and the core that it includes are withdrawn, depending on the type of the formation, the borehole may remain open or might collapse. An open borehole is a pathway that provides an opportunity for cross-contamination between various layers of the formation. For example, in the sampled formation, several distinct aquifers may be separated by layers of less permeable material, such as clay. In multi-layer aquifer systems, a higher aquifer, that is closer to the ground surface, is contaminated by material seeping in through the ground surface; the deeper aquifers are nonetheless protected by the separating layers of low permeability clay. An open borehole creates a pathway from the contaminated aquifer to the lower and pristine aquifers which will lead to their contamination over time. As another example, if a layer of PCB contaminated soil lies beneath a lake, the soil and sediment below this contaminated layer may remain unaffected. An open borehole creates a conduit for the contamination to be carried to the unaffected layers. Subsequently, each layer transports the contamination horizontally according to its own transport parameters. Evan collapsed boreholes are pathways because the formation is not going to return to its original layered form in the area of the creation and collapse of the borehole.
Note that contamination need not travel downward in a borehole. Lower aquifers may be at higher pressures and the contaminated groundwater of a lower aquifer can travel up an open borehole to an overlying aquifer. In underwater sediments, the water that fills the borehole provides a medium of transfer in both directions.
One conventional method of abatement of open boreholes is sealing the hole by filling it with a low permeability material such as bentonite, which is a form of clay. Bentonite is injected into the borehole, in the form of a slurry, after the sampling assembly is withdrawn from the hole.
Slurry injection is used for other purposes in drilling applications. Slurry injection may be used as a lubricant to facilitate drilling in hard to drill formations. Such formations are usually drilled using a rotary drill bit and slurry is injected as the drilling proceeds. In other applications, after the core casing has been withdrawn from inside a drill bit, and while the drill bit is still in the ground, a slurry tube is conducted down the drill bit to inject slurry into the borehole. Rotary drills have a corkscrew drill bit that is hollow to make room for a core casing inside. As the rotary drill bit makes progress into the soil, sampled core is also collected inside the core casing. The core casing may be withdrawn while the borehole is kept open by the drill bit. The slurry is injected before the hollow drill bit is retrieved and the hole gets a chance to collapse. In such applications, however, withdrawal of the core casing creates an open pathway that remains open before the slurry injection.